Method of producing ceramic articles and the articles produced by this method



United States Patent 3,219,466 METHOD OF PRGDUCING CERAMIC ARTICLES AND THE ARTICLES PRGDUCED BY THIS METHGD Thomas Henry lsherwood, Wolverhampton, England, as-

signor to Gibbons (Dudley) Limited, Dibdaie, Dudley, England, a British company No Drawing. Fiied Mar. 19, 1962, Ser. No. 189,994 5 Qlairns. (Cl. 166-58) This invention relates to ceramic articles and to ods of producing them.

The invention is primarily concerned with the produc tion of ceramic articles for industrial use in applications in which they are subjected to serve thermal loading either as to the magnitude of the temperature to which they are subjected or as to the severity of temperature change. Such temperature change may be severe because of the rate of change of temperature or because of the frequency with which it takes place or possibly because of both these factors.

Additionally the ceramic articles may be subjected to mechanical stresses. These may be fluctuating as for example where the articles are subjected to fluctuating bending, compressive or tensile loads. In some cases the stresses may arise through abrasion of the surface of the articles, such stresses then being in the nature of shear or tearing stresses applied locally to the surface.

These conditions are met either singly or in combination in ceramic articles incorporated in furnace and burner structures.

One of the specific objegts of the invention is to provide ceramic articles adapted :or use as load carrying members, for example when incorporated in floors, skids, or tracks of a furnace structure along or over which articles to be heated in the furnace are moved either by sliding motion which will give rise to both compressive and bending stresses of a fluctuating character as well as abrasive stresses or possibly by the rolling of carriages over ceramic articles formed as track components. This last mentioned application involves fairly severe compressive loading because of the small area of contact between the track element and the wheel or roller of the carriage, as well as a high frequency of fluctuation in cases where a plurality of carriages are traversed along the tracks at frequent intervals.

A further specific object of the invention is to provide ceramic articles suitable for incorporation in burner structures in furnaces. A specific part of a burner structure which is subjected to severe thermal loading is a burner block onto which the frame of a burner plays and which in consequence of this becomes heated to a high temperature to furnish radiant heat to the interior of a furnace structure. Yet another burner component which is similarly subjected to severe thermal loading is a burner sleeve which surrounds and defines the path of the combustible fuel and which is thus disposed in close proximity to or in contact with the flame yet which itself is often desirably made of relatively thin walled construction.

It is known that in materials employed for the formation of ceramic products and which are of crystaline form defect structures occur in these materials naturally on a small scale.

The atoms of the substance are disposed as is known in predetermined positions in a regular lattice. One defect structure known as a Frenkel defect is the disposal of certain of the atoms at interstitial posiitons in the lattice, that is to say in the spaces between the normal nodes of the lattice which such atoms would properly occupy.

Another defect structure known as the Schottky defect is one in which certain of the proper lattice positions or nodes are left vacant, that is not occupied by atoms, and

meththese atoms are situated at the surface of the crystalline structure.

The number of atoms so displaced from their proper positions in naturally occurring defect structures is very small.

Ceramic articles in accordance with the present invention and produced by the methods thereof are made up of crystalline materials in which a defect structure is deliberately contrived and on a substantially greater scale than occurs naturally in order to modify the mechanical characteristics of the article in a manner which renders it more resistant to mechanical and thermal loading of a severe character as hereinbefore specifically mentioned.

According to the invention broadly the method thereof comprises intimately mixing particles of a solid state substance in which oxygen is combined with another element with particles of a metal reactive autothermically with the oxygen containing substance, compacting the mixture to form it into the required shape, and supplying heat to the mixture at a sufiiciently high rate to initiate an autothermic reaction in which a non-stoichiometric compound is formed.

A ceramic article made in accordance with the invention broadly is an autothermically formed double oxide composition in which the oxygen is combined with different elements (usually metals or one of them may be silicon) in non-stoichiometric proportions so far as one at least of these elements is concerned and in many cases with respect to both these elements. The non-stoichiometry exhibited by the ceramic article arises from the presence of a defect lattice structure in which metal atoms are disposed in interstitial positions in the lattice or at the surfaces of the crystals, in the latter case thereby creating a vacancy in the proper node or lattice position normally occupied by the metal atom.

When these defect structures are produced on a scale substantially exceeding that upon which they occur naturally the mechanical and thermal properties of the articles thereby produced can be beneficially controlled and modified.

A further feature of the invention is the continuance of heat treatment in an appropriate environment after completion of an autothermic reaction between the main reaction substances in order to modify the defect structure, and hence the non-stoichiometry, in certain parts of the article, for example, the outer layers thereof in order to achieve heterogeneous mechanical and/ or thermal characteristics for the article as a whole.

For example, the outer layers of an article could be subjected to this after treatment to render them extremely hard and resistant to abrasion whilst the inner part or core of an article may be left with a higher degree of nonstoichiometry (occurring as displacements of metal atoms from normal positions( to provide beneficial mechanical properties of another kind, for example, a greater degree of elasticity which renders the article capable of with standing changing or fluctuating mechanical stresses.

Referring now generally to the selection of the constituents to be mixed together one constituent or several constituents used simultaneously in the mixture are selected from a primary group which consists of refractory metal oxides, refractory metal oxide minerals, or refractory compounds having chemical formulae of the general form X O X O in the anhydrous state. X and Y represent metals (or one of them may be silicon) and O designates oxygen in the usual way, the suffixes n, m, p and q have values satisfying normal valancy requirements.

The second constituent or constituents is selected from a secondary group consisting of metals.

Examples of materials in the primary group are titania, silica, alumina, chromium oxide, zinc oxides, iron oxide and calcium oxide all of which are refractory metal oxides.

Examples of refractory metal oxide minerals falling in the primary group are:

Chromite: One variety which has been successfully employed is Transvaal ore containing 40% or greater chromium oxide (Cr O 12% to 20% aluminum (A1 and 16% to 25% iron oxide (FeO).

Ilmenite: The variety which has been successfully employed is Malayan or Finnish ore, the main constituent of which is FeO.TiO

Brookite: The main constituent is titanium dioxide (TiO Magnesite: The variety which has been successfully employed is sea water or Grecian, the main constituent being magnesium oxide (MgO).

Mullite: The main constituent is aluminum silicate (3Al O .2SiO

Petalite: The main constituent is Al O .Li O.8SiO

Sillimanite: The variety successfully employed has been calcined kyanite or South African sillimanite (Al O .SiO

Clays: Of various compositions of which the principal constituent is hydrated aluminum silicate (Al O .2SiO .2H O) Examples of metals which form the secondary group are aluminum and magnesium.

The constituent from the primary group and the constituent from the secondary group are selected because of ability to undergo, when suitably initiated, an exothermic reaction in which the heat developed is sufficiently great to sustain the continuance of the reaction (herein referred to as being then autothermic) resulting in the reduction of the oxide, mineral, or compound, selected from the primary group and producing a double oxide, that is to say a compound in which the remaining oxygen is combined in part with the metal and in part with the metal or element with which it was formerly combined in the selected primary group constituent. Further, the selection is such that the double oxide compound so produced exhibits a defect structure and hence non-stoichiometry. One or both metals may have less oxygen combined with it than corresponds to the valency requirement.

In many cases the selected metal is different from the metal with which the oxygen is initially combined in the primary group but it is possible to react a primary group constituent with a metal which is the same as that with which the oxygen is combined in the primary constituent. An example of this is reaction between chromium and chromium oxide, possibly in the presence of magnesite (MgO).

It is emphasised that the autothermic reaction is not intended to produce and does not produce a type of substance known generally as a cermet, that is a mixed metal and non-metal refractory phase in which the free metal is disposed as a mixture between the particles of the nonmetallic refractory.

If reduction of the selected oxide should proceed so far as liberation of the metal from this oxide or should excess reacting metal be used, the existence of free metal as a mixed phase, that is to say outside the lattice structure of the crystalline double oxide, would represent only a temporary or intermediate stage in the production of the final article and such metal would be oxidised either in the later stages of the autothermic reaction, or in the after treatment.

One of the essential features of the method is that having selected the reacting constituents from the primary and secondary groups the autothermic reaction is initiated and caused to proceed at an unusually accelerated rate. This is achieved by the extremely rapid application of heat which may be conducted in the form of a plunge heating operation, that is to say where the reacting constituents are suddenly immersed in a furnace chamber already brought to an elevated temperature, or where a somewhat less but still accelerated rate of reaction is required, the temperature may be raised at much higher rates than those customarily employed in firing conventional ceramic articles which do not have defect lattice structures, except such as occur naturally in certain of the constituents.

Selection of the constituents from the primary and secondary groups is also made with a view to ensuring very highly exothermic reactions.

In addition to the reacting constituents the mixture may contain diluents selected to modify the rate of reaction as desired to provide the required body characteristics for the formed article, that is to say porosity or thermal characteristics. Such diluents may be materials which do not react with the selected autothermically reacting constituents. Alternatively, part of one of the autothermically reacting constituents may be present in coarser grained form in which it will not react so readily or vigorously and will thereby act as a diluent.

Referring now to specific examples illustrative of the practice of the invention ceramic articles in accordance therewith have been made as follows.

Example 1 The constituents of the mixture are as follows:

60 parts of aluminum powder mesh (by weight) 50 parts of titania runa (Laporte) (by weight) 200 parts of silicon carbide-l4 mesh (by weight) 10 parts of Petalite 100 mesh (by weight) 2% core oil 1.5% Celacol 2500 (a cellulose ether manufactured by British Celanese Limited and sold under the registered trademark Celacol) The material may be brought to the required state of division, i.e., grain size in any suitable manner as for example by crushing in a pan milll For ensuring thorough mixing of several constituents so that these are uniformly distributed throughout the mixture they are placed in a ball mill which comprises a rotary drum or cylinder containing heavy balls for further pulverising the material.

Upon mixture in this manner and addition of the bind ing materials, namely, the core oil and Celacol the mixture is pressed to the required shape, for example for making bricks of square or rectangular form as viewed in plane, side elevation and end elevation. It will be understood that such pressing is mentioned by way of example and any moulding or shaping technique applicable to materials in a plastic or pliable condition, yet to some degree shape-holding may be employed according to the particular form and nature of the article to be made.

It will be understood that the binding materials, namely, the core oil and Celacol, may be added to the mixture whilst this is being pulverised in the pan mill or further pulverized and mixed in the ball mill.

After drying by allowing the pressed or formed article to stand for a suitable period of time in an atmosphere which is either natural or deliberately dried or possibly subjected to gentle heating merely for the purpose of drying, the shaped article is plunge fired by introduction into a furnace which has previously been heated as for example by an electric heating element in the furnace chamber to maintain a furnace temperature of 1250 C.

The plunge firing initiates extremely vigorous autothermic reaction which takes place between the aluminum powder and the titania and also between the aluminum power and the petalite.

This reaction is self-sustaining and is allowed to proceed to completion. Thereafter the temperature of the furnace is reduced to 1000 C. and is held at this value for 10 hours this constituting the after treatment of the article.

During this after treatment an oxidising atmosphere is maintained Within the furnace chamber.

Articles produced in accordance with this example have an inner or core portion which exhibits a greater degree of non-stoichiometry and defect structure than do the outer layers of the article the modification of the structure in the latter case having been achieved by the after treatment.

Mechanically the core portion of the article has greater resilience than the outer layers, such resilience being manifested predominantly in one direction in consequence of the defect structure.

The outer layers still have some degree of elasticity greater than that which would be afforded by a stoichiometric composition of the same materials but it is less directional than that afforded by the core.

Further the outer layers are more highly resistant to abrasion, that is to say will resist shear or tear stresses to a higher degree than the core portion alone.

These render the article especially suitable for use as a burner block in furnaces where temperatures of up to 1600 C. are required to be developed.

Example 2 In this example the constituents are the same as those employed in Example 1 except that the Petalite is omitted.

Again the physical operations involved in mixing and shaping the article may be carried out as described in Example 1. The after treatment may be the same as specified in Example No. 1.

Articles formed in accordance with the present example are very highly resistant to rigorous conditions of temperature both as to high values of temperature, rate of temperature rise, and fluctuations thereof.

Articles formed in this way are suitable for use as burner sleeves, that is to say sleeves containing or enclosing the combustion chamber or part thereof of burners fed with gaseous or liquid fuels.

In some cases the after treatment was extended for 100 hours. A subsequent X-ray examination of the article however still disclosed that it was formed of aluminum titanate exhibiting defect structure with respect to the oxygen atoms. This indicates the fact that stable new compounds are formed in which oxygen deficiency is not made good or eliminated even by very prolonged soaking at elevated temperatures in an oxydising atmosphere.

Example 3 In Example 3 the constituents of the mixture were as follows:

Parts by weight Chromite calcined100 mesh 100 Aluminum powder 100 Pulverised fireclay '70 Bauxitel/3O mesh 250 Ilmenite 25 Celacol 2500 (2% solution), approximately 7.5%.

The mixing and shaping of the materials was carried out as in Example No. 1.

Some of the products made were of substantial size, namely, up to 80 pounds in weight.

Instead of plunge heating, the articles were subjected to a very rapid temperature rise to a value of 1350 C. in 3 hours during which time the autothermic reaction between the reacting constituents was completed. The reacting components in this case were the aluminum powder and Chromite, the aluminum powder and pulverised fireclay and the aluminum powder and ilmenite.

Most of the bauxite acted as a diluent although some of it may have entered into autothermio reaction with the aluminum powder.

The after-treatment in this case consisted of a soaking for a further 18 hours at 1350 C. and thereafter a further soaking for 24 hours at 1260" C. in the furnace after which the furnace chamber was allowed to cool naturally.

At all stages an oxidising atmosphere was maintained.

As a possible alternative to this procedure the temperature of the furnace was raised after placing the pressed and shaped constituents therein and after the initial 3 hour period wherein the temperature is raised to 1350 C. to a temperature in the range 1450 to 1500 C. This additional heating is stopped when cone 20 (Segcr) placed initially in the furnace chamber falls.

Articles produced in accordance with this example are suitable for use as skids, hearth blocks, or bottom wall blocks for supporting products which are moved slidably in or into a furnace chamber.

The outer layers of the block are highly resistant to abrasion due to modification of the defect structure and non-stoichiometry which is more pronounced in the central or core portion as described in connection with Example 1.

Example 4 The constituents of the mixture were as follows:

Parts by weight Bauxite, 610 mesh 10 Bauxite, 10-30 mesh 42 Bauxite, 40s mesh 11 Ilrnenite 10 Aluminum powder l1 Bentonite 2 Ball clay 6 Percent Liquid lye 2 Celacol, 2% solution 6 The constituents were prepared, mixed together, and shaped as described in connection with Example 1.

The articles were subjected to rapid heating in 3 hours by raising the temperature of a furnace in which they were placed to 1250* C. in which time the autothermic reaction was completed.

The reacting substances in this case were the aluminum powder and the ilmenite, the aluminum powder and the ball clay, and the aluminum powder with some proportion of the bauxite, the remainder of which especially the course grain particles, acted as a diluent.

The after treatment consisted of maintaining a temperature in the furnace at 1250 C. for 24 hours after which the furnace chamber was allowed to cool naturally.

As a somewhat less preferable alternative the articles have been fired in a continuous tunnel kiln. The total transit time through the kiln was 72 hours. The temperature gradient Within the kiln was such that articles reached a temperature environment of 700 C. after 6 hours travel from the entrance and a temperature environment of 1200 C. to 1300 C. after 48 hours from the entrance. The remainder of the travel (24 hours) taking place in a temperature environment cooling gradually to ambient temperature at the exit.

The atmospheric conditions maintained throughout in either case were oxidising.

Articles made in accordance with this example are capable of withstanding high local compressive stresses of a travelling or fluctuating character such as occur in rails or tracks along which roller equipped furnace carriages or trolleys travel and may advantageously be used for this purpose.

Articles formed in accordance with the foregoing examples also afford a high degree of dimensional stability under conditions of high temperature operation as well as resistive to chemical and physical attack.

Whilst the invention has been described in accordance with this foregoing specific example it will be understood that it is not so limited and that other selections can be made from the primary and secondary groups of substances to achieve the formation of ceramic articles having non-stoichiometric or defect structures.

Further in the course of the reaction intermediates 7 or new lattice structures in highly reactive states are produced which are not isolated or produced in normal techniques of firing ceramic articles. These new lattice structures are of temporary existence and are formed on a much more extensive scale than occurs in conventional firing of ceramic articles. Also owing to the exceptionally high temperatures developed 1600 C.- 1700 C. these new lattice structures or intermediates react vigorously to form new final compounds of stable form. These can lead to the production of new compounds with new characteristics.

For example, when aluminum is reacted with titania alone, or in the presence of calcined alumina, the compound formed is cubic aluminum titanate.

This new compound is present in articles formed in all the foregoing Examples 1 to 4 where titania is among the reacting constituents.

It has also been found that a cubic form of alumina (zeta) alumina, unknown in nature, is formed in the presence of the nitride a product of the burning of aluminum in the presence of air.

What I then claim is:

1. A method of forming a ceramic article comprising (a) intimately mixing particles of at least one solid state oxygen containing compound selected from a group consisting of titanium oxide, aluminum oxide, iron oxide, chromium oxide, magnesium oxide, and calcium oxide,

(b) with particles of a metal selected from a group consisting of aluminum and magnesium,

(c) compacting the mixture to form it into the desired shape of the article,

(d) supplying heat to the mixture at a suificiently high rate to initiate an autothermic reaction between the said metal and said selected oxygen containing compound to form non-stoichiometric compounds,

(e) and subjecting the article to an after-treatment upon completion of the said reaction comprising heating the resultant non-stoichiometric compounds in an oxidizing atmosphere at a temperature at least approximately 1000 C. for a period of time at least approximately 10 hours to effect modification of the degree of non-stoichiometry in an outer layer of the article relative to that existing in the interior of the article.

2. A method according to claim 1, wherein particles of a solid state diluent substance which do not enter significantly into the autothermic reaction are mixed with particles of the said selected oxygen containing compound and metal prior to the initiation of said reaction.

3. A method according to claim 1, wherein (a) the selected oxygen containing substance comprises titanium oxide, and

(b) the after-treatment comprises heating the article at a temperature in the region of 1000 C.

4. A method according to claim 1, wherein (a) the selected oxygen containing substance comprises chromium oxide, and

(b) the after-treatment comprises heating the article in an oxidizing atmosphere at a temperature in the region of 1300 C.

5. A method according to claim 1, wherein (a) the selected oxygen containing substances comprise titanium and iron oxides, and

(b) the after-treatment comprises heating the article in an oxidizing atmosphere at a temperature in the region of 1250 C.

References Cited by the Examiner UNITED STATES PATENTS 2,741,822 4/1956 Udy 106-55 2,872,726 2/1959 Goliber 106-65 3,049,432 8/1962 Weber 106-66 FOREIGN PATENTS 733,390 7/1955 Great Britain.

TOBIAS E. LEVOW, Primary Examiner.

JOHN H. MACK, Examiner. 

1. A METHOD OF FORMING A CERAMIC ARTICLE COMPRISING (A) INTIMATELY MIXING PARTICLES OF AT LEAST ONE SOLID STATE OXYGEN CONTAINING COMPOUND SELECTED FROM A GROUP CONSISTING OF TITANIUM OXIDE, ALUMINUM OXIDE, IRON OXIDE, CHRONIUM OXIDE, MAGNESIUM OXIDE, AND CALCIUM OXIDE, (B) WITH PARTICLES OF A METAL SELECTED FROM A GROUP CONSISTING OF ALUMINUM AND MAGNESIUM, (C) COMPACTING THE MIXTURE TO FORM IT INOT THE DESIRED SHAPE OF THE ARTICLE, (D) SUPPLYING HEAT TO THE MIXTURE AT A SUFFICIENTLY HIGH RATE TO INITIATE AN AUTOTHERMIC REACTION BETWEEN THE SAID METAL AND SAID SELECTED OXYGEN CONTAINING COMPOUND TO FORM NON-STOICHIOMETRIC COMPOUNDS, (E) AND SUBJECTING THE ARTICLE TO AN FTER-TREATMENT UPON COMPLETION OF THE SAID REACTION COMPRISING HEATING THE RESULTANT NON-STOICHIOMETRIC COMPOUNDS IN AN OXIDIZING ATMOSPHERE AT A TEMPERATURE AT LEAST APPROXIMATELY 1000*C. FOR A PERIOD OF TIME AT LEAST APPROXIMATELY 10 HOURS TO EFFECT MODIFICATION OF THE DEGREE OF NON-STOICHIOMETRY IN AN OUTER LAYER OF THE ARTICLE RELATIVE TO THAT EXISTING IN THE INTERIOR OF THE ARTICLE. 